Steam turbines are used in a variety of different applications, such as driving centrifugal compressors, electrical generators in power plants or the propellers of ships and submarines. Steam turbines operate by converting the heat energy of high pressure steam into rotational mechanical shaftwork. The steam turbine includes bladed wheel assemblies coaxially-mounted on a shaft rotating about an axis and enclosed by a housing to contain the high pressure steam. The bladed wheel assemblies have a plurality of buckets upon which the steam impinges to drive the shaft.
Steam turbines usually rotate at high rotational speeds, so that only small changes in mass distribution can have an effect on balance. For example, if one or more of the bladed wheel assemblies coaxially-mounted on the shaft moves slightly, the steam turbine can go out of balance. When the steam turbine is out of balance, the shaft vibrates and can bow and possibly break. Prior methods and devices for correcting imbalance problems are difficult to use, expensive, and time consuming.
For example, many steam turbines are designed with either holes drilled into the shaft adjacent to the bladed wheel assemblies or holes drilled into sleeves which are coaxially mounted on the shaft adjacent the bladed wheel assemblies. The holes are designed to receive weights to help balance the shaft. Long and narrow passages are provided in the housing, which surrounds the bladed wheel assemblies and shaft, to provide access to the holes so weights can be added for trim balancing of the steam turbine. Although adding weights helps to balance the steam turbine, accessing the holes through these passages is very difficult. Often a portion of the housing must be removed to get to the passages which lead to the holes in the shaft.
Other steam turbines, particularly older steam turbines, do not even have holes drilled in the shaft adjacent the bladed wheel assemblies or holes drilled in sleeves mounted adjacent to the bladed wheel assemblies in which weights could be added for trim balancing. As a result, the balancing operation for these steam turbines is even more difficult and time consuming. When the bladed wheel assemblies on the shafts in these steam turbines go out of balance, the steam turbine must be stopped, disassembled, and the parts of the steam turbine must be balanced in a balancing machine usually at an external facility to correct for any balancing problems.
Accordingly, there is a need for an apparatus and method to balance assembled steam turbines, without having to access balancing holes in the shaft or in sleeves on the shaft through long narrow passages in the housing or without having to perform major disassembly of the steam turbine.